wayland: Fix buffer age

We need to reset the age of the *new* front buffer to 1, not the old
one.

It turns out they're easy to mix up right when you're about to swap
them.

src/wayland/mod.rs

@@ -186,14 +186,14 @@ impl<D: HasDisplayHandle + ?Sized, W: HasWindowHandle> WaylandImpl<D, W> {
             .dispatch_pending(&mut State);
 
         if let Some((front, back)) = &mut self.buffers {
+            // Swap front and back buffer
+            std::mem::swap(front, back);
+
             front.age = 1;
             if back.age != 0 {
                 back.age += 1;
             }
 
-            // Swap front and back buffer
-            std::mem::swap(front, back);
-
             front.attach(self.surface.as_ref().unwrap());
 
             // Like Mesa's EGL/WSI implementation, we damage the whole buffer with `i32::MAX` if